Clay thin films have excellent flexibility, and have highly excellent gas and liquid barrier properties since the clay particles are densely oriented in layers (refer to Patent Document 1). Moreover, they are also excellent in terms of heat resistance and nonflammability. By taking advantage of these characteristics, it is expected that the clay thin films that are transparent can be used as a film substrate for liquid crystal and organic electroluminescence (EL) displays.
When the clay thin film is used as a film substrate for an organic EL display for example, a transparent conductive film followed by an organic EL device are laminated on the clay thin film. When the surface of the clay thin film is not flat at the time of lamination, the transparent conductive film cannot be formed uniformly causing reduction in electrical conductivity at the defective parts. Furthermore, the organic EL device is also damaged.
Since the transparent conductive films and organic EL devices are generally formed of thin-film layers that are extremely thin ranging from a few tens to a few hundreds of nanometers, their substrates are required to have excellent surface flatness, for example, a low surface roughness of a few to a few tens of nanometers.
The following method is known as a conventional method to produce clay thin films. Firstly, clay is dispersed in water or a liquid having water as a main component, which is a dispersion medium, to prepare a clay-dispersed solution. This dispersion is allowed to stand horizontally in order to slowly deposit clay particles and also to separate water or the liquid having water as a main component, which is the dispersion medium, by means of solid-liquid separation. Thereby clay forms a film, and thus a clay thin film can be prepared (refer to Patent Document 1). As a method to allow the dispersion to stand, a flat tray made of plastic or a metal is used and the dispersion is poured therein. However, in this case, the surface of the prepared clay thin film which is in contact with the tray has a different surface roughness to as opposed to that of its opposite surface (i.e. the interface with water which is the interface with air after the film is dried). That is, since the surface contacting the tray has a shape which reflects exactly the shape of the contacting tray surface, satisfactory surface flatness is achieved by using a tray with an excellent surface flatness. However, the surface opposite to the surface contacting the tray, that is, the interface with water which is the interface with air after drying, is highly uneven and satisfactory surface flatness is not achieved since it is formed solely by the sedimentation and deposition of clay particles.
[Patent Document 1]
Japanese Laid-Open Patent Application No. 2005-104133